1. Field of the Invention
This invention relates to a method for storing image signals, and an apparatus for carrying out the method.
2. Description of the Prior Art
Image filing apparatuses for filing image signals, which represent images, have heretofore been used in various fields. For example, at medical facilities, such as hospitals, many kinds of medical images are utilized for medical treatment or research. Most of the medical images are radiation images. In recent years, many computed tomography (CT) images, magnetic resonance (MR) images, and the like, are also utilized.
The medical images should be stored to permit investigation of changes in diseases or injuries of patients, and it is legally stipulated that the medical images must be stored for a predetermined period. Therefore, in hospitals, or the like, the number of stored medical images increases daily. The medical image have heretofore been stored in the form of hard copies. However, storage as hard copies requires a large storage space and burdensome operations for control and retrieval of the medical images in hospitals, or the like.
Accordingly, in recent years, an image filing apparatus has been proposed in which images, such as medical images, are retrievably stored (i.e. filed) as image signals on a storage medium. In cases where the medical images are filed in this manner on the storage medium, the space requirement and the burden of image storage can be reduced, and the images can be retrieved easily and quickly.
In an image signal representing an image, such as a medical image, in order for continuous tones to be expressed, many bits are assigned to each of the image signal components of the image signal representing the picture elements in the image. Therefore, the amount of the image signal representing each image is very large. Accordingly, usually, various reversible signal compression processing methods or various irreversible signal compression processing methods are carried out on image signals, and the amounts of the image signals are thereby decreased. Image signals, which have thus been compressed, are then stored on a storage medium.
With the reversible signal compression processing methods, an original image signal is compressed such that the original image signal can be restored completely from the compressed image signal. In such cases, an image can be reproduced from the restored image signal, which image has the same level of image quality as the original image. However, the amount of the original image signal cannot be decreased markedly, and the amount of the compressed image signal is comparatively large. Therefore, with the reversible signal compression processing methods, only a comparatively small number of images can be stored in the form of image signals on a storage medium having a predetermined storage capacity. Specifically, the reversible signal compression processing methods are not advantageous from the point of view of the image filing efficiency.
On the other hand, with the irreversible signal compression processing methods, an original image signal is compressed such that the original image signal cannot be restored completely from the compressed image signal. In such cases, a reproduced image is obtained from the restored image signal, which image has a lower level of image quality than the original image. However, the amount of the original image signal can be decreased markedly.
Therefore, in general, in image filing apparatuses for storing a large number of images in the form of image signals, which images should be stored for a long period, irreversible signal compression processing is carried out on original image signals, and the original image signals are thereby compressed to such extents that image signals representing images having acceptable levels of image quality can be restored from the compressed image signals. The compressed image signals, which have been obtained from the irreversible signal compression processing, are stored in the image filing apparatuses. In such image filing apparatuses, ordinarily, when an image signal is stored, a code signal is also stored simultaneously, which gives specifics about the image signal, such as the name of the image represented by the image signal and the date on which the image was recorded. The code signal serves as an index when the image signal, which has been stored, is read.
The image filing apparatus described above is provided with a function for carrying out irreversible signal compression processing on an image signal, and a function for storing a compressed image signal, which has been obtained from the irreversible signal compression processing, on a storage medium. The image filing apparatus is also provided with a function for storing a code signal, which gives specifics about an image signal, on a storage medium, and a function for reading a compressed image signal and a code signal, which have been stored.
Nowadays it is desired that not only can the information giving specifics about image signals be stored in the form of code signals, but information about handwritten characters, or the like, can also be stored as subsidiary signals giving specifics about the image signals such that it may be clear which subsidiary signals correspond to which image signals. In order for such a demand to be satisfied, it is thought that some of image filing apparatuses, which have heretofore been used, may be renewed such that information about handwritten characters, or the like, can also be stored. However, in order for some of image filing apparatuses, which have heretofore been used, to be renewed, considerable expenses will be required. Additionally, such new image filing apparatuses become incompatible with the image filing apparatuses, which have heretofore been used and in which only signals including no information about handwritten characters, or the like, are stored. Therefore, the system comprising the old and new image filing apparatuses becomes very inconvenient to operate.